A) Field of the Invention
The present invention relates to ZnO semiconductor and ZnO based compound semiconductor and further relates to a ZnO based light emitting device.
B) Description of the Related Art
Zinc oxide (ZnO) is direct transition type semiconductor having a band gap of 3.37 eV at a room temperature. Since a binding energy of exciton is as large as 60 meV, ZnO is expected as the material of a high efficiency light emitting device.
A high quality ZnO thin film can be formed on a ZnO substrate by epitaxial growth while preventing lattice mismatch. A ZnO substrate is formed by cutting a bulk crystal grown by, for example, hydrothermal synthesis.
A high quality ZnO thin film can be formed by molecular beam epitaxy (MBE). For example, ZnO is grown on a substrate by radiating, in an electrodeless discharge tube, an oxygen radical beam and a zinc (Zn) beam from a Knudsen cell (K cell) at the same time by using a radio frequency of 13.56 MHz to the substrate that has been heated to a growth temperature.
An epitaxial film of ZnO can be formed by setting the growth temperature Tg and flux ratios in a desired manner. A flux intensity of Zn is represented by JZn, and a flux intensity of O radical is represented by JO. A coefficient (Zn sticking coefficient) indicating a bonding feasibility of Zn to an O terminated plane of ZnO crystal is represented by kZn, and a coefficient (O sticking coefficient) indicating a bonding feasibility of O to a Zn terminated plane of ZnO crystal is represented by kO. In this case, a product of the Zn sticking coefficient kZn and Zn flux intensity JZn, kZn·JZn, corresponds to the number of Zn atoms bonded in a unit area of a ZnO substrate per unit time. A product of the O sticking coefficient kO and O flux intensity JO, kO·JO, corresponds to the number of O atoms bonded in a unit area of a ZnO substrate per unit time. The condition satisfying that the product kZn·JZn is equal to the product kO·JO is called a stoichiometry condition.
A flux ratio is defined by kO·JO/kZn·JZn (a ratio of a product of the O sticking coefficient and O flux intensity to a product of the Zn sticking coefficient and Zn flux intensity).
In a ZnO thin film forming process, kO·JO/kZn·JZn>1, i.e., the flux ratio being larger than the flux ratio of the stoichiometry condition, is called an O rich condition (film forming condition realizing O rich). kO·JO/kZn·JZn<1, i.e., the flux ratio being smaller than the flux ratio of the stoichiometry condition, is called a Zn rich condition (film forming condition realizing Zn rich).
A relation between the Zn flux intensity and the ZnO film growth rate on a Zn polarity plane (+c plane) is known. See “High-quality ZnO epilayers grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy” by Hiroyuki Kato, Michihiro Sano, Kazuhiro Miyamoto, and Takafumi Yao, Journal of Crystal Growth 265 (2004); pp. 375-381), which is hereby incorporated by reference in its entirety.
The growth rate of a ZnO film can be calculated by the following equation (1):G=[(kZn·JZn)−1+(kO·JO)−1]−1−RZnO  (1)where RZnO is a re-evaporation rate of ZnO. RZnO can be neglected at a growth temperature of 800° C. or lower.
The above-mentioned document reports the formation of ZnO films with various flax ratios, and found that reflection high energy electron diffraction (RHEED) images exhibit streak patterns only when ZnO films are formed under the extreme O rich condition (flux ratio of 5.6). It also reports that a flat surface profile (surface roughness of 2.9 nm) was obtained when measured by atomic force microscopy (AFM). It is also confirmed from these results that a ZnO thin film grows two-dimensionally.
In forming an n-type ZnO thin film to be used for a light emitting device, Ga, Al, In and the like are used as n-type impurities.